The present invention relates generally to a method and apparatus for testing polygon mirrors and more particularly to a method and apparatus for testing the angular inclination and flatness of the individual faces of a polygon mirror while the polygon mirror is rotating at its intended operating speed.
Polygon mirrors and well known in the art and widely used in optical reading systems, optical recording systems, optical printing systems and the like for producing a rotating beam of light. Polygon mirrors in which the mirror faces are adjustably attached to the sides of the polygon, such as by spring biased screws, are also well known in the art. Some of the advantages of polygon mirrors having adjustably attached mirror faces as opposed to mirror faces that are integrally formed on the sides of the polygon are that they are less expensive to manufacture, that the individual faces can be accurately adjusted to the proper angle and that if a face is or becomes scratched or unuseable for any reason it can be easily replaced without having to replace the entire polygon. Polygon mirrors having adjustable faces are especially useful in applications such as disclosed in U.S. Pat. No. 4,043,632 to Edwin A. Jeffery and Sigmond Hinlein wherein the individual mirror faces are inclined at different angles, which patent is incorporated herein by reference for its disclosure of a polygon mirror having adjustable faces.
For many applications it is necessary that the angular inclination, or pyramidal angle as it is commonly called, of the individual faces be accurate to within .+-.5 seconds of arc and that the flatness of the individual faces be within wavelengths. Therefore, it is important that polygon mirrors be tested to see if these parameters are within the acceptable limits. It has been found that there is usually some movement in the positioning of the mirror faces of a polygon mirror having adjustable faces when the polygon mirror is brought up to its intended operating speed. The movement may be the result of distortions produced by centrifugal force as the polygon rotates or by stretching or movement of the screws or bearings or springs that are used to attach the faces to the polygon. For example, a movement of a mirror face of about 5 millionths of an inch will produce an angular error in the order of about one second of arc on a face of one inch width. Consequently, in order to accurately test polygon mirrors having adjustable faces it is essential that the tests be conducted while the polygon mirror is rotating at its intended operating speed.
In the past, the testing of such polygon mirrors has been achieved by masking off every face of the polygon mirror except the one to be tested, bringing the polygon mirror up to its intended operating speed, illuminating the polygon mirror with a continuous beam of light and then observing the location of the deflected beam on a screen located at a large distance, such as for example 60 feet, from the polygon mirror. Because of the large distance between the polygon mirror and the screen, small angular errors can be easily observed. One of the disadvantages of such an arrangement is that it requires a relatively large amount of space and is hence cumbersome. Another disadvantage of such an arrangement is that because the screen is located at a large distance from the polygon mirror the system is subject to errors caused by air movement and errors caused by differential vibrations of the different mounting structures for the polygon mirror, the screen and the light source. Still another disadvantage of such an arrangement is that it requires physically masking off each face except the one being tested, bringing the polygon mirror up to operating speed to make the test readings and then stopping the polygon mirror so that the particular face that is masked can be changed. Yet still another disadvantage of such an arrangement is that it is generally useful only for testing for pyramidal errors.
Accordingly, the need exists for a new and improved technique for testing the faces of a polygon mirror while the polygon mirror is rotating at its intended operating speed and more particularly a technique that overcomes the disadvantages in the prior art technique enumerated above. The present invention provides such a technique.